zathura-pdf-mupdf (zathura support for mupdf backend)

The zathura-pdf-mupdf plugin adds PDF support to zathura by using the
mupdf rendering library (replacing the poppler-based PDF support from
the zathura package).

To switch between the mupdf and poppler backends without reinstalling
packages, adjust the symlink at /usr/lib(64)/zathura/libpdf.so. It
should point to either pdf/libpdf-mupdf.so or pdf/libpdf-poppler.so.
You can also reinstall zathura to switch to the poppler backend, or
reinstall zathura-pdf-mupdf to switch to the mupdf backend (whichever
was installed last "wins").
